liquidlight

My ex and I have been divorced for almost 3 years now and we have two children, ages 3 and 4. She has been remarried for roughly 18 months and is married to the man that is the reason why we divorced in the first place (a long story all by itself). Due to the circumstances of our seperation and attempted reconcilliation my kids have known this man for most of their lives, all of that being said the problem that I'm having is this: because they're now married my ex and her mother (who lives in the upstairs apartment over my ex) have taught my children to call him "Daddy Ben." I personally have a serious problem with this, if for nothing else because it upset me the first time I heard it and I asked her to not let them call him that any longer. That was over a year ago and it's become a complete habit for them at this point. Am I just being petty? And does anyone have any suggestions on how I should approach changing this issue if possible?